1. Field of the Invention
The present invention relates to electrical circuits. More particularly, the invention relates to a method and apparatus for printing an electrical circuit onto a substrate using electronic inks that, when cured, have electrical functionalities.
2. Related Art
The electronics, display and energy industries rely on the formation of coatings and patterns of conductive materials to form circuits on organic and inorganic substrates. The primary methods for generating these patterns are screen printing for features larger than about 100 μm and thin film and etching methods for features smaller than about 100 μm. Other subtractive methods to attain fine feature sizes include the use of photo-patternable pastes and laser trimming.
One consideration with respect to patterning of conductors is cost. Non-vacuum, additive methods generally entail lower costs than vacuum and subtractive approaches. Some of these printing approaches utilize high viscosity flowable liquids. Screen-printing, for example, uses flowable mediums with viscosities of thousands of centipoise. At the other extreme, low viscosity compositions can be deposited by methods such as ink-jet printing. However, low viscosity compositions are not as well developed as the high viscosity compositions.
Ink-jet printing of conductors has been explored, but the approaches to date have been inadequate for producing well-defined features with good electrical properties, particularly at relatively low temperatures.
There exists a need for compositions for the fabrication of electrical conductors for use in electronics, displays, and other applications. Further, there is a need for compositions that have low processing temperatures to allow deposition onto organic substrates and subsequent thermal treatment. It would also be advantageous if the compositions could be deposited with a fine feature size, such as not greater than about 100 μm, while still providing electronic features with adequate electrical and mechanical properties.
An advantageous metallic ink and its associated deposition technique for the fabrication of electrically electrical conductors would combine a number of attributes. The electrical conductor would have high conductivity, preferably close to that of the pure bulk metal. The processing temperature would be low enough to allow formation of conductors on a variety of organic substrates (polymers). The deposition technique would allow deposition onto surfaces that are non-planar (e.g., not flat). The conductor would also have good adhesion to the substrate. The composition would desirably be inkjet printable, allowing the introduction of cost-effective material deposition for production of devices such as flat panel displays (PDP, AMLCD, OLED). The composition would desirably also be flexo, gravure, or offset printable, again enabling lower cost and higher yield production processes as compared to screen printing.
Further, there is a need for electronic circuit elements, particularly electrical conductors, and complete electronic circuits fabricated on inexpensive, thin and/or flexible substrates, such as paper, using high volume printing techniques such as reel-to-reel printing. Recent developments in organic thin film transistor (TFT) technology and organic light emitting device (OLED) technology have accelerated the need for complimentary circuit elements that can be written directly onto low cost substrates. Such elements include conductive interconnects, electrodes, conductive contacts and via fills.
Existing printed circuit board technologies use process steps and rigidly define the printed circuit board in the context of layers. Only one layer of conductive material is permitted per layer due to the copper etch process used. In general, devices cannot be mounted on internal layers.